1. Field of the Invention
This invention relates to a recording medium having a porous organic resin layer on a base member and a method of manufacturing the same. The present invention also relates to an image forming method for forming images on such a recording medium preferably by means of an ink-jet recording method.
2. Related Background Art
Ink-jet recording systems are adapted to record images and/or characters on a recording medium (such as paper) by ejecting fine droplets of ink based on any of the currently known various operation principles. An ink-jet recording system provides advantages including high speed printing, a low noise emission level, adaptability to multi-color printing, versatile recording pattern forming capabilities and elimination of development and fixing processes and therefore has been finding an increasingly large number of applications. Furthermore, currently known multi-color ink-jet recording systems can form multi-color images with an image quality comparable to that of color images printed by using a color photography system at cost lower than ordinary color printing if the number of copies is relatively small. Therefore, they are being used also in the field of full color image recording.
While improvements have been made to ink-jet recording apparatus and ink-jet recording methods to meet the demand for a higher recording speed, a higher image definition and a higher full color image quality, such a rigorous demand has also been directed to the recording medium to be used for ink-jet recording. For the ink-jet recording system, ink containing an aqueous solvent that may be water or a mixed solution of water and an organic solvent to a large extent is normally used because ink droplets have to be ejected at high speed from nozzles toward the recording medium. For recording color images with a high color density, it is therefore necessary to use ink at a high rate. On the other hand, the beading phenomenon of combined and fused ink dots can appear to disturb the image printing operation because ink droplets are ejected continuously. To prevent the beading phenomenon, the recording medium to be used with the ink-jet recording system is required to absorb ink at a high rate and to a large extent.
Various forms of recording medium have been proposed to meet the above listed requirements. For instance, a variety of types of recording medium having thereon a porous layer containing inorganic particles (of alumina hydrate, etc.) have been proposed to improve the recording medium in terms of ink absorption, coloring and the resolution of the printed image. Such proposals include the one described in Japanese Patent Application Laid-Open No. 2-276670. On the other hand, Japanese Patent Application Laid-Open No. 4-101880 proposes various different types of recording medium prepared by forming an ink fixing layer of transparent resin that dissolves into liquid or swells by absorbing the solvent contained in ink.
Both Japanese Patent Applications Laid-Open Nos. 59-22683 and 59-222381 propose a recording medium having a layer made of thermoplastic resin particles, emulsion and latex formed on a base member. Japanese Patent Application Laid-Open No. 11-192775 describes a recording medium adapted to be used with pigment ink and invented by the inventor of the present invention.
Furthermore, Japanese Patent Application Laid-Open No. 9-99634 describes a recording medium having an ink receiving layer made of a polymer complex obtained by dissolving a binding agent containing a basic polymer and styrene/(meth)acrylic acid copolymer and organic particles and mixing them.
Japanese Patent Application Laid-Open No. 9-156211 describes a recording medium having an ink receiving layer that is formed on a transparent support member and made of fine particles of a crosslinked polymer having an average particle diameter of less than 200 nm and water-soluble resin, the recording medium showing a transmissivity per sheet of more than 80%.
Japanese Patent Application Laid-Open No. 10-324053 describes a recording medium having a porous film coat formed by using emulsion containing carboxycellulose nitrate and a film forming aid.
While the above described prior art provides various types of recording medium that are improved in terms of ink absorption, the resolution of the image formed on it, image density, transparency and gloss, they are accompanied by a number of problems that have arisen as a result of the remarkable recent improvements of recording apparatus that make it possible to print quality images comparable to those of silver photography at high speed.
For instance, while the recording medium having a porous layer of inorganic particles (of alumina hydrate, etc.) on the surface is advantageous in terms of the quality of the image formed on it and gloss, it can easily be damaged during transportation depending on the method of carrying it because the surface is highly vulnerable to damages. Additionally, when it is made transparent so as to be used as OHP film, it can produce its own shadow even if it is highly transparent and relatively free from haze because of the porous layer that is made of a combination of inorganic particles and organic resin. This problem may probably be due to the difference of refractive index between the inorganic particles, the organic resin and the pores. Still additionally, when pigment ink is used to print images on it, the pigment can form a deposit on the surface that is not absorbed by the porous layer because the pores of the porous layer is small relative to the pigment particles in the ink, whereby the surface smoothness becomes lost and the rub-off resistance is decreased. Furthermore, the recording medium can give rise to an additional problem of yellow discoloration that is specific to a recording medium carrying thereon a porous layer mainly made of such inorganic particles.
A recording medium having an ink fixing layer made of resin that dissolves into liquid or swells by absorbing the solvent contained in ink as disclosed in Japanese Patent Application Laid-Open No. 4-101880 is accompanied by a problem that the ink applied to it does not dry quickly but remains sticky after the application. Additionally, the ink receiving layer does not show a sufficient level of water-resistance and some of the ink applied to it can migrate when the dye of the ink is affected by moisture. Still additionally, cracks can occur in the printed areas (particularly in solidly printed areas) of the recording medium when a pigment is used as the coloring agent of ink because the ink receiving layer lacks water-resistance.
While a recording medium comprising an ink receiving layer made of thermoplastic resin particles, emulsion and latex formed on a base member as disclosed in Japanese Patent Application Laid-Open No. 59-22683 and 59-222381 shows a high ink absorption rate, the absorption of ink relies only on the gaps separating the thermoplastic resin particles and hence a thick ink receiving layer has to be used to obtain a sufficiently high ink absorption ability. Then, there arises a problem of reduced transparency and strength of the film layer. While Japanese Patent Application Laid-Open No. 11-192775 proposes an improved recording medium that is highly adaptable to the use of pigment ink, it has a configuration based on a concept that is totally different from the present invention because it has to be subjected to a heat treatment after the image forming process.
In the case of a recording medium having an ink receiving layer made of a polymer complex of a combination of a binding agent and organic particles as described in Japanese Patent Application Laid-Open No. 9-99634, pores formed in the ink receiving layer are not satisfactory for achieving a high ink absorption rate because the binding agent and the organic particles are dissolved in solvent firstly and then the obtained polymer complex is used to form the ink receiving layer.
It is also difficult to satisfactorily raise the ink absorption rate of a recording medium having an ink receiving layer that is formed on a transparent support member and made of fine particles of a crosslinked polymer having an average particle diameter of less than 200 nm and water-soluble resin as disclosed in Japanese Patent Application Laid-Open No. 9-156211 because the water-soluble resin is used as binding agent and hence it is impossible to provide sufficiently large gaps among the fine particles of the polymer and additionally because water-soluble resin is used at a ratio between 1:1 and 1:10 relative to fine particles.
Finally, in the case of a recording medium having a porous film coat formed by using emulsion containing carboxycellulose nitrate and a film forming aid as disclosed in Japanese Patent Application Laid-Open No. 10-324053, while gaps are satisfactorily formed among the fine particles of the emulsion, it is difficult to regulate the pH value in the porous film coat layer and, if the pH value is low and the layer is acidic, the dye contained in the ink can agglomerate in a peculiar way to alter their original tints after the printing operation.
In view of the above identified technological problems of the prior art, it is therefore the object of the present invention to provide a recording medium that is improved in terms ink absorption rate, non-stickiness, transparency, ability for fixing pigment ink, prevention of cracks in printed areas and color reproducibility for dye ink and satisfactory for forming images in terms of image density, gradation, water-resistance and ink fixation, as well as a method of manufacturing such a recording medium and a recording method adapted to record images on such a recording medium.
According to the present invention, the above object is achieved by providing a recording medium comprising a base member and at least a porous organic resin layer formed on the base member;
the porous organic resin layer including organic fine particles having both hydrophilic radicals and hydrophobic radicals, a water-absorbing binder and voids;
the porous organic resin layer showing a pore size distribution having the highest peak found within the pore radius range between 3 nm and 300 nm, a pore volume of 0.2 cm3/g or more and a pH value of 5.2 or higher as observed by a method conforming to JIS P 8133.
According to the invention, there is also provided a method of manufacturing a recording medium having a base member and at least a porous organic resin layer formed on the base member, the method comprising:
a step of forming the porous organic resin layer by applying to the base member and drying a coating formulation of aqueous dispersion containing organic fine particles having both hydrophilic radicals and hydrophobic radicals, a water-absorbing binder and a basic substance and showing a pH value of 5.2 or higher as observed by a method conforming to JIS Z 8802.
According to the present invention, there is also provided an image forming method for forming an image by applying ink to a recording medium according to the invention, the method preferably using an ink-jet recording method for applying ink to the recording medium.